1. Field of the Invention
The invention relates generally to aircraft designs, and, more particularly, to unmanned aircraft designs that combine the features of fixed wing aircraft and vertical takeoff and landing (VTOL) aircraft.
2. Description of the Related Art
Tactical Unmanned Aerial Vehicles (UAVs) have revolutionized the way wars are fought and intelligence is gathered because of their low-cost, safety, and long endurance. However, one major limitation of current UAVs is the need for prepared launch and recovery sites with large footprints or a traditional airport. A portable system that does not require bulky launch equipment or runways would greatly increase the utility of manned and unmanned aircraft, much as helicopters do for low endurance operations.
Launch and Recovery Equipment (LRE) for UAVs can be comprised of both a launch vehicle as well as a recovery system, such as a large net. This equipment has many operational burdens. First, the LRE site must offer a clear launch and approach path. This path must be obstacle-free over a mile or more beneath a 5 degree flight path, which makes urban basing very difficult. Additionally, some UAVs need two dedicated technicians per shift to handle LRE operations. LRE hardware typically weighs 1,500-3,000 pounds for the launcher, and a similar amount for recovery equipment. These units must be transported to the launch site, which often requires two or more Humvees for transport. Similarly, traditional long range or high endurance manned aircraft need a runway with the attendant clearings below the takeoff and landing flight paths.
Previous aircraft designs attempt to combine the vertical takeoff and landing (VTOL) and hover capabilities of a helicopter with the increased speed and range capabilities of fixed wing aircraft. These hybrid designs reduce the footprint necessary for launch and recovery but are more complex than either helicopters or conventional take off and landing aircraft as they generally incorporate multiple propulsion systems, each used for a different flight mode. These designs can include “tail sitter” configurations, so named because the aircraft takes off and lands from a tail-down orientation. Other designs can include “nose sitter” configurations, so named because the aircraft takes off and lands from a nose-down orientation.
One example of a nose-sitter design includes a VTOL hybrid, which includes a conventional propeller for fixed wing flight and a folding rotor near the tail of the aircraft. These designs may have high hover efficiency; however, they also require complex mechanical systems and weigh more than other designs due to the requirement of two separate propulsion systems, one for each flight mode.
Other VTOL designs can include “tail sitter” configurations, so named because the aircraft takes off and lands from a tail-down orientation. Conversion from vertical to horizontal flight for these hybrid designs typically requires a configuration change and dedicated engines for each configuration. Prior solutions that combine VTOL and cruise performance compromise performance in both flight modes.
A VTOL airplane or UAV that uses the same propulsion for both flight modes would have many structural benefits, including reduced complexity and weight of the launch equipment and ease of operation in more remote locations, as well as numerous mission benefits that are enjoyed today by helicopters. These include hover-and-stare in urban-canyons and sit-and-stare for extended silent surveillance. Further, sit-and-wait operation allows the airplane or UAV to be pre-deployed to a forward area awaiting mission orders for remote launch of the aircraft. Upon receiving the mission order, the vehicle can launch without leaving any expensive launch equipment at the launch site.
Some existing VTOL designs suffer from poor endurance and speed. Forward flight efficiency may be improved by partial conversion to an aircraft like the V-22 but endurance issues remain. Many VTOL aircraft also require a high power-to-weight ratio. These aircraft may be used for high-speed flight if the aircraft is fitted with a special transmission and propulsion system. However, achieving high endurance requires efficiency at very low power. Thus the challenge exists to create a virtual gearbox that equalizes power and RPM for VTOL and fixed wing flight achieving highly efficient cruise with the benefits of a vertical takeoff and landing configuration.